Presence, distribution and physiological function of adrenergic and muscarinic receptor subtypes in the human heart

The sympathetic and parasympathetic nervous system play a powerful role in controlling cardiac function by activating adrenergic and muscarinic receptors. In the human heart there exist α1-, β1- and β2-adrenoceptors and M2-muscarinic receptors and possibly also (prejunctional) α2-adrenoceptors. β1- and β2-adrenoceptors are quite evenly distributed in the human heart while M2-receptors are heterogeneously distributed (more receptors in atria than in ventricles). Stimulation of β1- and β2-adrenoceptors causes increases in heart rate and force of contraction while stimulation of M2-receptors decreases heart rate (directly in atria) and force of contraction (indirectly in ventricles). Pathological situations (such as heart failure) or pharmacological interventions (for example, β-blocker treatment) can alter the distribution of β1- and β2-adrenoceptors in the human heart, while M2-receptors are only marginally affected. On the other hand, relatively little is known on distribution and functional role of α1- and α2-adrenoceptor subtypes in the human heart.